It's not a joke

During the revolutionary war, there was a play put on by the British in Boston called THE BLOCKADE. In his book 1776, David McCullough describes the scene of the nights production: 

"A ridiculous figure, supposed to be George Washington, stumbled on stage wearing an oversized wig and dragging a rusty sword. At the same moment, across the bay, Connecticut soldiers led by Major Thomas Knowlton launched a surprise attack on Charlestown, and the British responded with a thunderous cannon barrage. With the roar of the guns, which the audience at Faneuil Hall took to be part of the show, another comic figure, a Yankee sergeant in farmer garb, rushed on stage to say the rebels were "at it tooth and nail over in Charlestown." The audience roared with laughter and "clapped prodigiously", sure that this, too, was part of the fun. 

"But soon finding their mistake [wrote an eyewitness] a general scene of confusion ensured. They immediately hurried out of the house to their alarm posts, some skipping over the orchestra, tamping on fiddles', and in, in short, everyone making his most speedy retreat, the actors (who were all officers) calling out for water to get the paint an smut off their faces, women fainting. etc."

As the British amused themselves, mocking the American army, the Americans were actually fighting the British. They were so full of themselves laughing at their enemy, they hadn't expected the enemy to actually fight them. And when they truth finally came to them, they perceived the warnings were all part of the show. The man who had real and vital news proclaimed his message with fervor, but because he was dressed as a clown, when he spoke, he spoke as a clown.